The present invention is in the field of mechanical testing and specifically relates to apparatus for applying a known axial force to a valve stem or to a shaft, the ends of which are not accessible.
At numerous points in a nuclear power plant, the flow of a fluid is controlled by large gate valves. When a considerable pressure difference exists between opposite sides of the gate, frictional forces become large and a large force must be exerted by the valve stem to move the gate. Under emergency conditions the pressure difference may reach very large values, and there is concern as to whether the valve will operate or can be operated safely under those conditions. In particular, there is concern about the force on the valve stem or other components.
FIG. 1 shows the type of valve on which the present invention is used. The operator (actuator) is connected to the valve by a yoke, and the valve stem is located between the two legs of the yoke. Obviously, the force in the valve stem is equal to the sum of the force in the two legs, but opposite in direction. It was well-known, even before U.S. Pat. No. 4,805,451, to install a strain sensor on one of the legs of the yoke to measure its elongation, from which the stress in the valve stem can be calculated based on knowledge of the size and material of the legs of the yoke.
With so much at stake, it would be highly desirable to provide for an independent check of the calculated force in the valve stem. This can be done by calibrating the strain sensor(s) mounted on the leg(s) of the yoke, or other sensors mounted at appropriate points on the valve. The calibration consists in applying a known force to the valve stem and noting the corresponding reading(s) of the strain sensor(s).
Once the reading of the strain sensor has been found for known amounts of force applied to the valve stem, there is a basis for believing that a particular reading of the strain sensor implies a particular amount of force on the valve stem.
In 1985 the U.S. Nuclear Regulatory Commission issued Generic Letter 85-03 which required operability verification of certain safety-related motor operated valves. Results, or lack thereof, from that investigation prompted the Commission to issue in 1989 Generic Letter 89-10 which required extensive verification, through in-depth accident condition testing, of a considerable larger number of motor operated valves. However, accident condition testing, i.e., configuring a plant system (piping, pumps, valves, and controls) to simulate accident conditions is costly, potentially hazardous and, in some cases, virtually impossible to perform without jeopardizing plant equipment integrity. In addition, current regulations propose follow-up testing, perhaps at periodic intervals, according to these same difficult accident conditions. Presently, there is no known method of satisfying all GL 89-10 requirements without actually generating system accident differential pressures across the valve and attempting to cycle the motor operated valve to demonstrate capability.
U.S. Pat. No. 4,891,975 to Charbonneau et al. discloses a prior art method for testing of motor operated valves under simulated loading conditions to determine a power parameter associated therewith. The power parameter is then measured during valve in use. The power parameter measured during imposition of the load is correlated with the power parameter measured during valve in use to provide an indication of thrust load developed in the valve operator during valve in use.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.